Three-piece, wound balls with balata (trans-polyisoprene) covers are typically preferred by professional and low handicap amateur golfers. These balls provide better golfers with a combination of distance, high spin rate, and control that is not available with an ionomer cover or in one-piece and two-piece balls. However, balata cuts easily, and lacks the durability required by the average golfer.
Two-piece golf balls, which are used by the average amateur golfer, provide a combination of durability and maximum distance that is not available with balata covered balls. These balls comprise a core formed of a single solid sphere, which typically comprises a polybutadiene based compound, encased in an ionomer cover, such as SURLYN.RTM.. These balls are extremely durable, and are almost impossible to cut. However, the durability results from the hardness of the ionomer, which gives such balls a very hard "feel" when struck with a golf club that many golfers find unacceptable.
Manufacturers have attempted to form blends of ionomers with other, softer polymers to soften the ball and improve the feel and the spin rate. Ionomers are ionic copolymers of an olefin and an unsaturated carboxylic acid in which at least a portion of the carboxylic acid groups have been neutralized with a metal ion. As a result, ionomers are immiscible with many other, softer polymers in the same manner as oil and water. A highly polar ionomer will not blend properly with either non-polar polymers or with polar, but non-ionic polymers that are sufficiently different chemically from ionomers that strong interactions between the polymers and the ionomers are not present in the mixture.
A mixture of polymer components is immiscible unless strong interactions are present between the polymer components in the mixture, such as those that exist between polymers with similar polarities. It is only when these strong interactions are present between the polymer components in the mixture that the polymers are miscible. As a result, mixtures of ionomers and non-polar polymers or of ionomers and polar polymers that cannot form strong interactions are immiscible.
A blend of miscible polymers is homogeneous on a microscopic scale, and consists of a single, continuous phase. In contrast, separate phases are formed in mixtures of immiscible polymers. Typically, the polymer that is present in the lower concentration forms a discontinuous or discrete phase, and the polymer that is present in the higher concentration forms a continuous phase. Mixtures of polymers that are highly immiscible are heterogeneous on a macroscopic scale, as well as on the microscopic scale. Golf balls formed with mixtures of polymers that are highly immiscible, such as prior art balls having a cover formed from a blend of an ionomer and a thermoplastic urethane, lack durability, and crack or split on impact.
Polymers are not always totally miscible or totally immiscible. Blends of such polymers are heterogeneous on a microscopic scale, but may be homogeneous on a macroscopic scale. Immiscible blends that are homogeneous on a macroscopic scale are referred to as being compatible, and often provide commercially attractive polymer blends that frequently possess enhanced physical properties when compared to the constituent polymers. Compatible blends form two phases that do not have well-defined boundaries, as each polymer can partially penetrate the phase of the other polymer. This interfacial interaction results in the improved properties of the compatible blend. Immiscible blends can also be compatibilized by the addition of a molecule that is at least partially miscible in each of the two phases, so that the interfacial interaction between the polymers in the two phases is improved.
Attempts have been made to overcome the deficiencies of blends of ionomers with other, softer polymers that would be expected to be immiscible or incompatible. For example, U.S. Pat. No. 5,397,840 to Sullivan et al. discloses blends of "ionic" and "non-ionic copolymers". Although it would be expected that blends of ionic and non-ionic polymers would be immiscible, both the "ionic copolymers" and "non-ionic copolymers" disclosed by Sullivan are highly polar, and, thus, strong interactions are present between the polymer components in the blend, which presumably result from "ion-hopping" during the blending stage. Therefore, the polymers in these blends are miscible, or at least compatible. The "ionic copolymers" are ionomers, consisting of a copolymer of an .alpha.-olefin and an .alpha.,.beta.-ethylenically unsaturated carboxylic acid, where at least part of the acid groups are neutralized by a metal derivative. The "non-ionic" copolymers are simply copolymers or terpolymers of ethylene and acrylic or methacrylic acid or propylene and acrylic acid, i.e., an .alpha.-olefin and an .alpha.,.beta.-ethylenically unsaturated carboxylic acid, in which the carboxylic acid groups are not neutralized. Blending such a "non-ionic" polymer with one or more ionomers would likely result in an exchange of acidic protons from the carboxylic acid groups of the "non-ionic" polymers and metal ions from the neutralized carboxylic acid groups in the ionomers, resulting in a blend of ionomers, which would be expected to be miscible.
U.S. Pat. No. 5,359,000 to Hamada et al. describes a golf ball cover material formed from a heated mixture of an ionomer resin, a copolymer of maleic anhydride or a polymer modified with maleic anhydride, and a metal hydroxide, metal carbonate, or metal acetate. As with U.S. Pat. No. 5,397,840, there are strong interactions between the components in the blend, so that the mixture of polymers appear to be miscible.
U.S. Pat. Nos. 5,098,105 and 4,986,545 to Sullivan disclose golf ball covers that include blends of ionomers and thermoplastic rubber elastomers modified with a polar group, wherein the latter material is selected from the group consisting of a maleic anhydride modified ethylene propylene copolymer, a maleic anhydride modified styrenic block copolymer, and a maleic anhydride modified ethylene-vinyl acetate copolymer.
European Patent Application 0 557 069 describes a golf ball cover containing a heated mixture of an ionomer resin and a polymer containing a glycidyl group.
In each of the patents discussed above, one or more ionomers are blended with a non-ionic polymer, containing groups such as maleic anhydride and glycidyl methacrylate that provide strong interactions between the ionomeric and non-ionic polymer components in the mixture. Therefore, the ionomers and non-ionic polymers are at least partially miscible, and compatible blends can be formed without a compatibilizer. None of the patents discussed above discloses immiscible blends of ionomers and polymers that require a compatibilizer to produce useful golf ball core, cover, or mantle layer compositions.
Gersappe et al., The Use of Graft Copolymers to Bind Immiscible Blends, Science, 265 (1994), describe copolymers with side chains that associate across boundaries between phase-separated regions in polymer blends to bind the regions together, and to enhance the structural integrity and mechanical properties of the resulting material. Normally incompatible mixtures of deuterated poly(ethyl acrylate) ("d-PEA") and deuterated poly(methyl methacrylate) ("d-PMMA"), compatibilized with PEA-polystyrene and PMMA-polystyrene are disclosed. There is no disclosure of compatibilized blends of ionomers and other, softer polymers that would normally be incompatible.
U.S. Pat. No. 5,321,089 to Cadorniga et al. describes a golf ball cover, containing ethylene-methyl acrylate, blended with an ionomer resin and a compatibilizer. The compatibilizer consists of a copolymer or terpolymer containing 80 to 95 percent by weight of an olefin, preferably ethylene, 20 to 5 percent by weight of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid, preferably acrylic or methacrylic acid, and possibly containing a small amount of an n-alkyl acrylate, such as n-butyl acrylate. The disclosed balls had durability properties superior to balata covered balls, failing in a hit test after only 145 to 182 hits. This is significantly less durable than golf ball covers made from ionomer blends.
None of these compositions provide the combination of durability and distance that is provided by two-piece balls with ionomer covers and the high spin rate and control that is available with a three-piece, wound balls with balata covers. Therefore, there remains a need for a method of making golf ball cores and covers that provides for the blending of thermoplastic ionomers with other, non-ionic polymers that are normally incompatible with ionomers to provide a one-piece, two-piece, or three-piece, wound golf ball with the durability and distance of a SURLYN.RTM. covered two-piece ball and the feel, click, and control of a balata covered three-piece ball. The present invention provides such a method, and produces such a product.